Cu-CHA 沸石中双核 Cu 活性位点的结构和反应活性，用于化学计量部分甲烷氧化制甲醇,ACS Catalysis

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Cu-CHA 沸石中双核 Cu 活性位点的结构和反应活性，用于化学计量部分甲烷氧化制甲醇
ACS Catalysis ( IF 11.3 ) Pub Date : 2024-02-21 , DOI: 10.1021/acscatal.3c06181
Laura N. Wilcox ₁ , Jose Rebolledo-Oyarce ₂ , Andrew D. Mikes ₁ , Yujia Wang ₃ , William F. Schneider _{2,

3} , Rajamani Gounder ₁

Affiliation

与铜离子交换的硅铝酸盐沸石在化学计量氧化和还原循环中促进部分甲烷氧化（PMO）生成甲醇，但活性铜位点的身份和反应机制的细节仍然存在争议。在这里，我们使用高对称性菱沸石（CHA）沸石框架作为模型支持来探讨本体组成、铜形态以及对各种氧化和还原处理的响应之间的关系。密度泛函理论和第一原理热力学与统计模型相结合表明，铜的形态和成分在很大程度上取决于铝的构型和外部气体条件。合成 Cu-CHA 样品以调查 Si/Al 和 Cu/Al 组成空间和骨架 Al 接近度的广阔区域。在暴露于不同氧化条件下时，通过原位X 射线吸收和紫外可见光谱进行表征表明，Cu 氧化程度对活化条件敏感，因此动力学和热力学因素都会影响给定材料中的 Cu 氧化性。CO 还原过程中的类似特征表明，CO 滴定 Cu ²⁺的量表明存在 O- 和 O ₂ - 桥接物质。相比之下，CH ₄和自还原 (He) 处理减少的 Cu 位点数量与 CO 类似，但数量较少，这表明 O ₂桥接的 Cu 二聚体是前一种还原途径中潜在的常见中间体。化学计量 PMO 循环中甲醇产量（每铜）的系统性增加随着双核 O _x桥接铜位点的比例的增加而增加，这表明这些物质是活性位点，如更新的 PMO 反应机制中所述。

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Structure and Reactivity of Binuclear Cu Active Sites in Cu-CHA Zeolites for Stoichiometric Partial Methane Oxidation to Methanol

Aluminosilicate zeolites exchanged with copper ions facilitate partial methane oxidation (PMO) to methanol in stoichiometric oxidation and reduction cycles, yet the identities of active Cu sites and details of the reaction mechanism remain debated. Here, we use the high-symmetry chabazite (CHA) zeolite framework as a model support to probe the relationship between bulk composition, Cu speciation, and response to various oxidizing and reducing treatments. Density functional theory and first-principles thermodynamics combined with statistical models reveal that Cu speciation and composition depend strongly on Al configuration and external gas conditions. Cu-CHA samples were synthesized to survey broad regions of Si/Al and Cu/Al composition space and framework Al proximity. Characterization by in situ X-ray absorption and UV–visible spectroscopy during exposure to different oxidation conditions reveal that the extent of Cu oxidation is sensitive to activation conditions and thus that both kinetic and thermodynamic factors influence Cu oxidizability in a given material. Similar characterizations during CO reduction reveal that CO titrates Cu²⁺ in amounts suggesting the presence of both O- and O₂-bridged species. In contrast, CH₄ and autoreduction (He) treatments reduce similar but smaller numbers of Cu sites than CO, implicating O₂-bridged Cu dimers as a potential common intermediate in the former reduction pathways. A systematic increase in methanol yields (per Cu) in stoichiometric PMO cycles increase with the fraction of binuclear O_x-bridged Cu sites suggests these species as active sites, as depicted in an updated PMO reaction mechanism.

更新日期：2024-02-21

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